ESIPT and anti-Kasha behavior in hydroxy-aza-[n]cycloparaphenylenes

Abstract

We explore the excited-state intramolecular proton transfer (ESIPT) phenomenon in hydroxy-substituted aza-[5]-cycloparaphenylene (HA-[5]CPP). Computed excitation energies show well-separated S1 and S2 states, with the latter possessing a relatively higher oscillator strength. Upon photoexcitation to S2, the molecule undergoes an intramolecular proton transfer over a barrierless pathway to relax at the proton-transferred tautomeric form, demonstrating anti-Kasha photochemistry. Further, well-separated S1 and S2 state minima of the tautomers allow a direct fluorescence emission from S2, thus making it a potential anti-Kasha emitter.